Lenses for eyeglasses typically are prepared by a laboratory by customizing lens blanks which are prepared by the manufacturer. These stock lens blanks come in standard sizes and shapes and the laboratory must grind and polish the blanks to the proper prescription and to the shape required for insertion into a particular style of frame.
Although the precise method of preparation of the lenses varies from laboratory to laboratory and with different types of lenses, there are certain techniques which are commonly used by many laboratories, and these will be described hereinbelow.
In most laboratories, tape is used to grasp the lens blank during grinding. The lens blank is first placed in a tape applicator machine with a convex side being exposed. Tape is then adhered to the convex side of the lens blank. The tape, is drawn onto the convex side of the lens by a vacuum while the lens resides in the machine. The use of the vacuum ensures that the tape is tightly bonded to the lens, and that it is stretched over the lens to conform to its shape. This stretching process provides a tighter bond and removes any wrinkles or other imperfections in the tape. This tape is typically made of a plastic material, such as polyethylene, and can be stretched upon the application of pressure to conform to the shape of the surface of the lens blank and grasp it tightly. A typical example of such a tape can be purchased from Venture Tape Corp., under the product designation Blocker Tape. With the tape tightly stretched over the lens blank, a circular ring is clamped over the lens, pressing the tape tightly against the lens surface and stretching the tape to conform to the shape of the lens surface. Using the inside surface of the ring as a guide, excess tape is then cut away so that the tape does not extend beyond the outer perimeter of the lens blank. The lens, with the tape attached to its convex surface, is removed from the machine.
The lens blank is then placed in a blocking machine in which a low-temperature metal alloy is melted and injected into a mold in direct contact with the tape. Typically, The alloy used melts either at a temperature of about 117.degree. Fahrenheit or of about 155.degree. Fahrenheit. This alloy is then allowed to cool. The alloy is of such a nature that it forms a mechanical bond with the tape when cooled. The mold is then removed to reveal a metal block which is formed of the alloy and which is bonded to the tape on the lens blank.
The block with the lens blank secured thereto is then mounted in a generator, and the concave side of the lens blank is ground to the desired prescription. Thereafter, the block is mounted again in a polishing machine in which the concave side of the lens blank is polished. The block may also be manually held for further grinding of the edges of the lens blank and polishing thereof.
When all of the grinding and polishing operations on the lens blank have been completed, a deblocking ring is used to remove the block. The lens blank is rapped while in the blocking ring until the block falls off through a hole, after which the block may be melted and reused. Thereafter, the tape is peeled off the lens blank. Finally, the lens blank is edged to shape it to fit into a frame selected by the user.
For certain lenses, it is necessary to provide visual markings on the lens blank so that the laboratory technician knows how the lens is to be mounted in the frame and so that the lens is properly oriented when the lens is sized and shaped to fit into the frame after it has been ground to a prescription. In the absence of these visual markings, the lens may be improperly edged or edged in such a way that the lens is mounted in the frame out of proper alignment. This problem is particularly acute with so-called progressive lenses in which the prescription changes progressively from top bottom. In progressive lenses, there are no visually identifiable features which allow proper orientation of the lens, such as are found in conventional bifocal lenses. Bifocal lenses display a readily observable discontinuity, which allows one to determine the top and bottom of the lens. Thus, it is impossible to determine which is the top or bottom of a progressive lens. These progressive lenses would be useless if they were mounted sideways so that the prescription changed from side to side or were mounted upside down so that the portion of the lens used for reading was at the top and that used for long distance viewing was at the bottom.
Typically, the lens blanks are marked with visible ink indicia by the manufacturer to allow proper alignment of the lens. This ink, however, must be removable after the lens has been mounted, so that it does not interfere with viewing. Thus, the ink cannot be etched into the lens blank. Also, the ink must be disposed on the convex side of the lens blank, since all of the grinding occurs on the concave side. Such grinding would obviously remove any markings placed there by the manufacturer. However, the ink used on the convex side, if not etched into the lens blank, typically is removed by the tape after the grinding process has been completed. It is at this time when the markings are actually needed, so that the lens can be properly oriented.
One solution to this problem was to use ink which is visible only under ultraviolet light and which is etched into the lens. Therefore, after removal of the tape, the lens can still be properly oriented by the laboratory technician using ultraviolet light. The technician typically views the lens in ultraviolet light, and places a temporary marker on the lens which is aligned with the etched ink indicia, and which is visible in ordinary light. This marker is then removed when the lenses are inserted into the frames. However, this solution typically requires a large amount of the technician's time to precisely align the marker on the lens under ultraviolet light. This extra step significantly increases the cost of the lens as well as decreases the production rate of the laboratory. The marker used typically is a type of vinyl which clings to glass, such as static cling vinyl. Such a marker remains aligned on the lens during the edging operation, but can be quickly removed from the lens when finished without leaving any residue and without damaging the lens.
It is therefore an object of the present invention to eliminate the added step of placing a visible marker on the lens after the grinding operation has been completed.
It is another object of the present invention to provide a method and apparatus for allowing the use of visual markings on the lens which are visible in ordinary light which remain after the grinding process for proper alignment of the lens, but which can be removed quickly and readily once the lens has been mounted in the frame.
It is a further object of the present invention to reduce the time required for grinding and mounting of lenses in an eyeglass frame.
It is yet another further object of the present invention to provide a method and apparatus which permits the use of markings on a lens for alignment of the lens which are visible in ordinary light but which can be readily removed when necessary, which method and apparatus use existing apparatus and do not require significant changes in the present methods and apparatus used for grinding lenses.